The present invention generally relates to a method for improving the control characteristics of automobiles and more particularly relates to using the vehicle""s hydraulic brake system with a closed-loop control system for improving vehicle stability
It can be observed in automotive vehicle control systems known in the art and used at present for anti-lock control (ABS) or driving stability control (ESP, ASMS, DDC) that with an initial pressure (pressure in the master cylinder) which is only slightly in excess of the wheel lock pressure level, a relatively poor (i.e. below the theoretically attainable value) brake performance is achieved because the control algorithms and control operations which aim at an optimum braking effect require an optimal, i.e. high initial pressure, as a precondition, or they are dimensioned for high initial pressures. To take into account and detect situations with low initial pressures which lie only slightly above the wheel lock pressure level, it is known in the art to implement defined learning algorithms which accordingly modify pressure modulation in response to relatively low initial pressures. However, a certain period of time lapses until these learning algorithms are optimally built up, with the result that precious braking effect is wasted and the stopping distance becomes longer than necessary.
In WO-A-97 13670 a method of operating a hydraulic brake system is described which assumes a brake system that includes brake slip control, traction slip control, and yaw torque control. The brake system shall be appropriate for active braking. The term xe2x80x9cactive brakingxe2x80x9d exclusively refers to a traction slip control operation or a yaw torque control operation by brake intervention, and brake intervention occurs without the driver""s intent, i.e. without application of the brake pedal.
WO-A-96 02409 discloses a method of operating a wheel-lock controlled automotive vehicle brake system for driving stability and/or traction slip control operations. The objective of this method is to ensure prefilling of the wheel brakes with pressure fluid in order that the wheel brakes can be acted upon with braking pressure according to driving stability and/or traction slip control, although the driver himself/herself does not apply the brake pedal.
Finally, German patent application No. 39 35 595 describes a brake system for controlling a wheel brake of a vehicle wherein a separate magnetically-operated two-position/three-way valve in each circuit of the brake system is used to control pressurization of the respective brake circuit. The application further discloses a method-of preventing an imminent locked condition of the rear wheels.
Experience shows that the xe2x80x98normal driverxe2x80x99 applies the brake only hesitantly in many situations so that an initial pressure which is sufficient for an optimal controlled braking operation is not achieved at all, or at a too slow rate.
An object of the present invention is to eliminate the difficulties which occur especially at relatively low initial pressures, i.e. initial pressures which lie only slightly above the wheel lock pressure level, and to improve the control operation in such situations by enhanced utilization of the prevailing coefficient of friction or road surface conditions, thereby increasing the braking effect.
The method of the present invention achieves the objectives by implementing the following method. Upon commencement of the control, that branch of the braking pressure control system to which the wheel detected as unstable is connected is temporarily uncoupled from the master cylinder pressure or initial pressure by closing the pressure fluid conduit in the direction from the wheel brake to the master cylinder of the brake system, and that by returning pressure fluid from the accumulator (low-pressure accumulator) into the branch to which the unstable wheel is connected, a relatively quick rise of the braking pressure in the control branch or, respectively, a quick approach of the wheel braking pressure to the wheel lock pressure level is caused (xe2x80x98quickxe2x80x99 meaning: compared to the braking pressure rise without uncoupling).
Even at a relatively low initial pressure, the method of the present invention permits achieving a pressure increase and an approach of the pressure in the mentioned branch and, thus, also in the wheel that became unstable to the wheel lock pressure level, which is relatively quick in comparison to conventional systems.
Upon commencement of the control, the separating valve is closed and the pressure fluid required for pressure build-up is initially taken from the accumulator (low-pressure accumulator), whereupon the special valve will be opened, i.e. switched to open passage, only after the accumulator has been emptied at least in part.
It has proved especially suitable to measure the degree of filling of the accumulator or the quantity of pressure fluid stored in the accumulator or to determine them in approximation by producing a so-called volume model, and to control the opening of the special valve in dependence on the degree of filling or the pressure fluid quantity. It is important that the fluid volume required for the pressure build-up in the wheel brakes is at disposal on the suction side of the hydraulic pump when it is needed. This fluid volume is either taken from the accumulator or supplied from the master cylinder by way of the special valve. Thus, for example, the special valve will only be opened or switched over to open passage as soon as the degree of filling or the volume in the accumulator has fallen below a predetermined minimum value.
In another embodiment of the present invention, the current pressure which is instantaneously prevailing in the master cylinder, is compared to the current deceleration of the vehicle or a pressure value which corresponds to. the deceleration, respectively. As is known, there is a defined relation between the pressure and the vehicle deceleration which can be achieved due to this braking pressure. When the comparison between pressure and deceleration shows that the vehicle deceleration exceeds a plausible value which corresponds to the current pressure in the master cylinder, this indicates that the vehicle brakes at a higher rate than the driver desires. The special control of the present invention xe2x80x98oversteersxe2x80x99, or there is an error. Therefore, switch-back to a standard braking pressure control is effected where the pressure upstream of the inlet valves to the wheel brakes is coupled to the initial pressure.